The purpose of the present investigation was to determine the types of ventricular arrhythmias induced during programmed electrical stimulation (PES) and the relation of induced ventricular tachycardia to incidence to spontaneous arrhythmia and presenting symptoms in 95 patients with hypertrophic cardiomyopathy (HCM). Sustained VT was induced in 38 (40%) of patients. This consisted of rapid polymorphic VT in 60%, relatively slower monomorphic VT in 32% and ventricular fibrillation in 8% of patients. There was correlation between inducibility of VT and severity of presenting symptoms (sudden cardiac death more than syncope more than presyncope more than asymptomatic) and occurrence of VT on Holter monitoring. These findings suggest that PES truly identifies patients with cardiac electrical instability and severity of symptoms and presence of spontaneous VT may both be markers of this condition